We use standard formulae to calculate the resonant opacity (e.g. [1]}, [2]}, [3]}, [4]}, [5]}). Since we deal with temperatures much lower than what corresponds to H \(\alpha \) , we do not include stimulated emission, and approximate the ground state to be dominantly populated, which implies that the partition function is \(Q(T)\approx 2\) . With this approximation, we do not need to handle the well-known divergence of \(Q\) , which, however, can be corrected easily by the occupation probability formalism [6]}. We assume a Doppler line profile, appropriate for our temperature regime.
